1. Field of the Invention
The invention relates to a method and an apparatus for bonding inner leads of lead frames to electrodes of semiconductor chips through wires, and more particularly to pictorial recognition of bonding sites at which inner leads are bonded to electrodes of semiconductor chips.
2. Description of the Related Art
In a conventional method of recognizing inner leads, there has been employed a charge coupled device (CCD) camera for recognizing bonding sites located in the vicinity of distal ends of inner leads of a lead frame, and then there has been calculated coordinate values for the bonding sites in a certain coordinate system. Then, a wire-bonding head wire-bonds inner leads to electrodes of a semiconductor chip mounted on a lead frame through wires in a subsequent stage in accordance with the thus calculated coordinate values.
FIG. 1 is a flow chart showing respective steps of a conventional method. In step 1, inner leads of lead frames successively fed are recognized by a CCD camera in binary or multivalued condition. Then, in step 2, coordinate values in X and Y axes are calculated for bonding sites in a certain coordinate system. Thus, there are obtained coordinate values of bonding sites necessary for wire-bonding a plurality of inner leads to a single semiconductor chip.
Then, in step 3, wire-bonding is carried out in accordance with the coordinate values of bonding sites calculated in step 2. Steps 1 to 3 are repeated until all of inner leads are wire-bonded to electrodes of semiconductor chips, as indicated in step 4.
As is obvious in view of the flow chart in FIG. 1, all of inner leads have been recognized by a CCD camera to calculate coordinate values of all of bonding sites, and then wire-bonding has been carried out in the above mentioned conventional method.
A lead frame having the small number of inner leads often has relatively wide inner leads, and hence dispersion in a position of inner leads can be absorbed in such a wide width of inner leads. Hence, wire-bonding can be carried out in accordance with bonding site data in advance stored in a memory, even if bonding sites in inner leads are not recognized by, for instance, a CCD camera. However, a lead frame having the greater number of inner leads is often made to have small width inner leads or to have complicated shape. Such small width inner leads and complicated shape causes deformation of inner leads when a lead frame is formed, and/or lowers accuracy with which a lead frame is formed. Thus, it was necessary in a conventional method to recognize all of bonding sites in inner leads prior to wire-bonding.
It takes about 2 to 5 seconds per a semiconductor chip for recognition of all of bonding sites. Hence, if four semiconductor chips are to be mounted on a lead frame, it would take 8 to 20 seconds for accomplishing recognition of all of bonding sites in such a lead frame. Thus, if recognition of all of bonding sites in a lead frame is to be carried out, it is not possible to shorten fabrication time, resulting in that the recognition is a bar for improving productivity.